This invention relates to an improved, highly reliable activator capable of initiating the crystallization of a super-cooled aqueous salt solution contained within a flexible container, to thereby cause the contents to become hot and to form a heat pack.
In recent years there has been an increasing interest in the provision of heat packs which utilize a super-coolable aqueous salt solution that can be activated to liberate heat but is also rechargeable and re-usable many times. Such heat packs have significant advantages over former means such as a hot water bottle or a heated electric element because of their portability and performance upon a moments notice without the need for a supply of hot water or electricity to generate the heat. A super-cooled heat pack in its liquid state can be transported readily and is always available on a moments notice to provide localized heat, as for the relief of pain and soreness in parts of the human body. Heat packs are becoming more popular with the growing population of persons who engage in exercise, athletes, and sportsmen, and are particularly useful in hospitals, clinics and in the home for the convenient generation of localized heat to provide relief to the patient.
Prior art devices were described as early as 1978, such as in U.S. Pat. No. 4,077,390, which describes a flexible container filled with a super-cooled aqueous salt solution and also containing a flexible ferrous metal strip characterized by one or more fissures or slits which are said to initiate crystallization of the solution when the metal strip is flexed. The fissure or slit is so prepared that the opposing sides of the fissure do not touch or scrape one another but ultimately taper to and terminate in a blind end. The patentee expresses his belief that the flexing of the metal piece produces minute continuances or extensons of fracturing of the metal at the blind end of the slit, and that these new fractures are instrumental in initiating crystallization. While the patentee calls for a ferrous metal strip he actually specifies stainless steel as the metal.
Heat packs made in accordance with the teachings of U.S. Pat. No. 4,077,390 prove to have serious drawbacks in practical use. The continual bending to initiate crystallization can create a break in the activator strip along the lines of the slits or cuts. Once broken the effectiveness is lost and the heat pack becomes unusable. Furthermore, the broken metal can cause a puncture in the heat pack and undesirable leakage of the salt solution.
Of even greater disadvantage is the fact that the activator strip is not wholely reliable to cause an activation of the crystallization. In such case the flexing of the metal strip fails to activate a reaction, even after many flexings. It can be seen quite readily that failure of only a small percentage of the heat packs to achieve initiation is certain to cause a loss of confidence in the unit and resulting lack of continuing repeat sales. The disappointed user very likely will return the failed heat pack for a refund with resulting loss to the manufacturer. Something approaching 100% reliability on the part of the activator strip to initiate crystallization is a virtual necessity for success in the marketing and use of the pack.
U.S. Pat. No. 4,379,448 recognized the shortcomings of the aforesaid heat pack and attempted to modify the problem by placing slits on the inside of the trigger with none of them extending to the marginal edges of the strip. Further, the patentee employs the concept of a snap-displacement of the metal, the so-called "oil can" effect when the metal is subjected to flexing or bending.
Once again the heat pack failed to perform with sufficient reliability in actual use since there was a percentage of the units which failed to activate despite any amount of flexing of the metal part. The same patentee tried an alternative construction in U.S. Pat. No. 4,460,546 wherein he introduced pinhole openings in the metal strip in place of the fissures or slits shown in his previous patent. This patentee introduced a non-ferrous metallic strip of a Berylium-copper alloy or phosphor-bronze, in order to avoid the deficiencies of the stainless steel trigger employed in earlier constructions. Heat packs made in this way likewise still were not completely satisfacotry. The "oil-can" effect can result in snap-deflection accidentally if the heat pack is handled too roughly. In other instances no amount of bending or snap action can cause the activation of the contents of the heat pack.
Still another attempt was made in U.S. Pat. No. 4,572,158 to resolve the problem of failures by making a least one major cut extending completely through the metal part and positioned totally within the exterior perimeter thereof, and making the slit wide enough so that the opposed facing edges are always spaced one from the other. Then the patentee made a second minor slit at one end or the other of the major slit and at an angle to the major slit. Again the minor slit is formed with two opposing facing edges that are minutely spaced one from the other so that they do not touch during manipulation. This patent also employed the snap displacement or "oil-can" effect.
The patentee explained his theory that the actual triggering or starting of the crystallization reaction is caused by the exposure of a new metal surface to the solution. His configuration is said to utilize the minute tearing of the metal surface at the end of the minor slits during flexing to thereby expose new metal to the solution and initiate crystallization. This arrangement has not proved to solve the ultimate problems of failures because in at least a significant percentage of the units repeated flexing is never successful in initiating the crystallization, or it my work one or a few times but then unaccountably fails to respond to flexing.
It should also be recognized that the attempt to exert quality control on the manufacture of the activator strips is a very costly and time consuming activity because of the minute size of the cuts and further because it is not possible to properly test the activator without putting it in a super-cooled salt solution and flexing it to see if activation actually occurs. Quality control after the heat packs have been assembled is also very expensive and never quite 100% satisfactory because an activator may work satisfactorily once or twice and then fail. To carry out the quality control to this level of testing requires repeated crystallization and melting, which requires the expenditure of much time and labor.
Thus, until the present invention was made the problem of unreliable activation of crystallization in heat packs still persisted and has had an inhibiting effect on the use of such units.